The present invention relates to a positioning apparatus which can move a work, for example, in a chamber insulated from the external environment.
In a semiconductor manufacturing apparatus, when working a work, the work is moved while it is placed on a stage within a process chamber maintained at a vacuum atmosphere or at a specific gas atmosphere. Here, in case where a positioning apparatus including a drive source is disposed within the process chamber, because the interior of the process chamber can be kept sealed with respect to the exterior of the process chamber, the vacuum atmosphere or specific gas atmosphere can be maintained relatively easily.
However, in case where the positioning apparatus including a drive source is disposed within the process chamber, the process chamber itself increases in size. This takes a lot of time to make the interior of the process chamber reach a given level of pressure, requires a large quantity of specific gas for filling the interior of the process chamber, or makes it difficult to maintain the positioning apparatus.
On the other hand, in case where the capacity of the process chamber is minimized, the above-mentioned problems can be truly solved but there is required a structure in which the table for placing the work thereon can be driven from the outside of the process chamber. As a structure of this type, there is known a structure in which there is disposed a moving shaft extending between the interior and exterior of the process chamber through an opening formed in the wall of a box body in communication with the process chamber, and the moving shaft is moved with respect to the box body to thereby drive the table existing in the interior of the process chamber from the exterior of the process chamber. And, there is also known another structure in which there is disposed a flat plate for shielding the surface of the opening of the box body, and the flat plate is moved with respect to the box body to thereby drive a table existing in the interior of the process chamber and placed on the flat plate from the exterior of the process chamber. In either of these structures, a clearance between the box body and moving shaft or flat plate is sealed by a differential pumping seal to thereby be able to maintain the environment of the interior of the process chamber (for example, see U.S. Pat. No. 4,191,385).
By the way, referring to the base material of the above-mentioned moving shaft or flat plate serving as a moving member, for example, for a vacuum atmosphere, in some cases, there is used metal material such as stainless steel or aluminum alloy (which has received surface treatment such as washing in order to reduce the gas release speed) which is excellent in vacuum characteristic. Also, for example, in case where the interior of the process chamber is held at a negative pressure, the moving shaft or flat plate is caused to receive a great force due to a difference in pressure between the interior and exterior of the process chamber; and, in case where the moving shaft or flat plate is flexed or deformed, there is a fear that the positioning accuracy of the work can be lowered. To avoid this, there is proposed an attempt to use ceramic material light in weight (low in specific gravity) and high in rigidity as the base material of the moving shaft or flat plate. Since the ceramic material is non-magnetic material, especially when an ion charge apparatus or an electronic beam apparatus requiring a vacuum environment and a low magnetic field variation is used to work a work existing in the interior of the process chamber, it can be said that the ceramic material is suitable for the moving shaft or flat plate.
However, because the normal ceramic material is sintered micro-particles (powder), due to occlusion of gas molecules (adsorption of gas molecules within a capillary tube), when compared with metal material, a large quantity of gas is released. Further, when compared with metal material, it is difficult to enhance the surface roughness of the ceramic material (in the case of the normal ceramic material, the limit is Ra=approx. 100 nm) and the actual area (adsorption area) of the ceramic material is larger than that of the metal material in case where they are equal in surface area to each other; and, as a result of this, a large quantity of gas is released. Therefore, in case where the surface of the ceramic material, which has adsorbed the molecules of the air, advances into the interior of the process chamber together with the movement of the moving shaft or flat plate, the accumulated air molecules are eliminated from the ceramic surface, which raises a fear that the pressure in the interior of the process chamber can be raised or the density of the specific atmosphere gas can be varied. Also, in case where, instead of the ceramic material, metal such as stainless steel is used as the blank material of the moving shaft and flat plate, it is true that the surface roughness of the metal is better (that is, the surface area is smaller) than the ceramic material, but, in the case of metal atoms as well, there occurs such phenomenon as in the ceramic material, that is, adsorption of the molecules of the air; and, therefore, there is a possibility that there can be raised a similar problem.
However, even in case where there is used the metal material that, as described above, is excellent in vacuum characteristic, in the portion of the moving member that is exposed to the air, adsorption of gas molecules to the surface of the exposed portion cannot be avoided. Normally, in a vacuum apparatus, there can be obtained a good vacuum condition through the following exhaust process, comprising: a step (1) of releasing the air from the interior of the vacuum apparatus; a step (2) of releasing gas molecules adsorbed to the inner wall of the vacuum apparatus; a step (3) of diffusing hydrogen gas from the composing members of the vacuum apparatus; and, a step (4) of allowing the air to permeate into the interior of the vacuum apparatus from the atmospheric environment.
However, in the conventional apparatus to which the differential pumping seal is applied, the surface of the exposable portion of the moving member, from which the adsorbed gas molecules have been released in a vacuum environment, is allowed to move from the process chamber through the seal portion thereof to the atmospheric environment. Therefore, since the surface of the exposable portion with the adsorbed gas molecules reduced in the vacuum environment, is exposed to the atmospheric environment, the adsorbed gas molecules are returned to their initial states. And, in case where the exposable portion returns into the process chamber, the adsorbed gas molecules of the exposable portion are gradually released in the interior of the process chamber; and, this operation cycle is repeated. The repetition of this operation cycle deteriorates the degree of vacuum in the interior of the process chamber.
While the foregoing description has been given on the assumption that the process chamber is held at a vacuum atmosphere, the phenomenon occurring on this assumption can be similarly found also in an apparatus in which a differential pumping seal is used in a high-density specific atmospheric gas process chamber. That is, there is a fear that gas molecules existing in the exterior of the process chamber can move into the interior of the process chamber along with the movement of a moving shaft or a flat plate to thereby vary the density of the gas existing in the interior of the process chamber.
Here, in the above-mentioned conventional positioning apparatus, there are found the following problems. Firstly, when the moving member is moved into and out of the process chamber kept in the vacuum environment, the gas particles adsorbed to the surface of the moving member in the exterior of the process chamber are released into the vacuum environment of the interior of the process chamber to thereby change the degree of vacuum in the interior of the process chamber. As a measure to avoid this problem, it can be expected to reduce the area of the surface of the moving member which is moved into and out of the process chamber. Here, generally, the surface of the moving member is worked with a certain degree of accuracy in view of the fact that it is guided by a guide member. However, to work the surface of the moving member with a higher degree of accuracy in order to reduce the surface area increases the cost of the moving member greatly. Therefore, this measure is not desirable. On the other hand, as a measure to cope with the change in the degree of vacuum, it can be expected to cover the surface of the moving member movable into and out of the process chamber with inert gas. In this case, however, it is necessary to dispose a storage tank for storing the inert gas therein, which unfavorably increases the cost of the positioning apparatus. Also, it is normally necessary to dispose a facility for collecting the inert gas and a facility for detecting the leakage of the inert gas.